1. Field of the Invention
The present invention relates generally to optical information recording media and, more particularly, is directed to an optical information recording medium in which a protective film having hygroscopic property is formed on a transparent substrate in such a manner that it can be stripped therefrom.
2. Description of the Prior Art
In an optical information recording medium in which a signal can be recorded and/or reproduced through a transparent substrate, a laser beam is focused on a recording surface from the transparent substrate side, thereby effecting the recording of an information signal. If a light incident surface of the transparent substrate is scratched or smudged, then a laser beam incident on the recording surface is hindered by scratches, dust or the like so that, upon recording, pits can not be formed on the recording surface accurately. Consequently, it is frequently observed that a recorded signal can not be reproduced in high fidelity. In particular, in an optical information recording medium in which a transparent substrate is made of resin, such as, polycarbonate or the like, static electricity is produced on the light incident surface of the substrate so that this surface is easily smudged by dust and so on. Further, if the light incident surface of the transparent substrate is rubbed by something very hard or shocked from the outside, the surface of the transparent substrate is easily scratched and damaged, making the above-mentioned problem more serious.
In order to protect the light incident surface of the transparent substrate from being smudged by dust or in order to protect the surface of the transparent substrate from being damaged by friction or shock from the outside, the following optical information recording media are proposed. These previously-proposed optical information recording media are disclosed in Japanese Utility Model Laid-Open Gazettes Nos. 60-60019, 57-49734 and 58-49337.
Japanese Utility Model Laid-Open Gazette No. 60-60019 proposes a magneto-optical recording medium in which a transparent plastic film cover is bonded only around a center hole of an optical recording medium such as a compact disc or the like by means of a bonding agent. In this recording medium, however, the transparent plastic cover is bonded only around the center hole of the optical recording medium so that the whole surface of the recording medium cannot be protected from dust and the like completely.
Japanese Utility Model Laid-Open Gazette No. 57-49734 proposes an optical recording medium in which a protective film is formed on the surface of a transparent protective layer of the optical recording medium so that it can be stripped therefrom. In this recording medium, however, the above-mentioned protective film is not treated to be protected from static electricity caused by dust. There is then a possibility that dust adhere to the light incident surface of the optical recording medium due to static electricity caused when the protective film is stripped from the surface of the transparent protective layer.
Japanese Utility Model Laid-Open Gazette No. 58-49337 proposes a recording medium in which a transparent film is formed through a bonding layer on a static electricity preventing layer formed by metalizing indium oxide so that the transparent film can be stripped from the static electricity preventing layer. This recording medium needs the static electricity preventing layer and the bonding layer, thereby increasing the manufacturing-process of the recording medium more than necessary.